Emulsion polymerization is the most important industrial method for manufacture of aqueous dispersion polymers. Emulsion polymerization is typically performed in an aqueous medium in the presence of a surfactant and a water-soluble initiator and usually rapidly produces high molecular weight homo or copolymers at high solids content and low dispersion viscosity. Its application requires the emulsification of the monomer in a medium, usually water, through the use of emulsifiers. These are supplied in addition to the other ingredients that go into most polymerizations, such as the initiator and chain transfer agents. The use and type of emulsifier determines many of the characteristics of the produced polymer or copolymer, which is typically a latex (stable colloidal suspension of polymer particles in a continuous phase, usually water). Moreover, the emulsifier usually cannot be completely removed from the latex. For this reason, and because of the great unpredictability of the efficacy of a given surface-active agent as an emulsifier in polymerization, many compounds that would theoretically be useful are not.
It is also known that emulsion polymerization requires the use of a surfactant to form a stable emulsion of monomers and to prevent coagulation of the product polymer. Surfactants are generally categorized into two types: either non-polymerizable, or polymerizable, that is co-polymerizable with the monomers for polymer formation. Surfactants are also categorized as anionic, cationic, non-ionic or zwitterionic depending on their chemical makeup. A problem which has arisen with the use of non-polymerizable surfactants is that they remain as a residue in the product polymer and, as they can be extracted by water, they make the product sensitive to water.
The name “emulsion polymerization” is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are stopped from coagulating with each other because each particle is surrounded by the surfactant (‘soap’); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a ‘hairy layer’ around a particle that repels other particles, because pushing particles together would involve compressing these chains.
Emulsion polymerization is used to manufacture several commercially important polymers. Many of these polymers are used as solid materials and must be isolated from the aqueous dispersion after polymerization. In other cases the dispersion itself is the end product. A dispersion resulting from emulsion polymerization is often called a latex (especially if derived from a synthetic rubber) or an emulsion (even though “emulsion” strictly speaking refers to a dispersion of an immiscible liquid in water). These emulsions find applications in adhesives, paints, paper coating and textile coatings. They are finding increasing acceptance and are preferred over solvent-based products in these applications as a result of their eco-friendly characteristics due to the absence of VOCs (Volatile Organic Compounds) in them.
Advantages of emulsion polymerization include:
High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate.
The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate.
Since polymer molecules are contained within the particles, the viscosity of the reaction medium remains close to that of water and is not dependent on molecular weight.
The final product can be used as is and does not generally need to be altered or processed.
The resulting latex is typically used in coating applications such as paints, stains, etc. Once the latex-containing product has been applied to a surface as part of a protective or decorative coating, the surfactant is no longer needed. In fact, the presence of the surfactant often degrades the moisture sensitivity of the coating. Other coating properties such as adhesion to the substrate surface can be negatively affected as well. This is largely due to the mobility of the surfactant polymers. For example, locally high concentrations of surfactant molecules can form in the coating from the coalescence of surfactant-coated micelle spheres. When the coating is exposed to water, these unbound surfactant molecules can be extracted from the coating leaving thin spots or pathways to the substrate surface. This can result in a pinholing effect and attack of the substrate by water.
Reactive surfactants contain a polymerizable moiety that can participate in free-radical emulsion polymerization reactions. When used in an emulsion polymerization a large fraction of the surfactant molecules become irreversibly bound to the emulsion polymer chains and droplets. When the latex is then incorporated into a coating such as paint, there is much less free surfactant to interfere with the desired coating properties or to reduce adhesion to the substrate.
A number of reactive nonionic and anionic surfactants are commercially available, including polyoxyethylene alkylphenyl ethers, sodium allyloxy hydroxypropyl sulfonates, alkenyl-functional nonionic surfimers, allyl methoxy triethylene glycol ether, sodium methallyl sulfonates, sulfopropyl acrylate, vinyl sulfonate, vinyl phosphate, monosodium ethylsulfonate monododecyl maleate, sorbitol acrylate, sorbitol methacrylate, perfluoroheptoxy poly(propyloxy)methacrylate, phenoxyl poly(ethyleneoxy acrylate, phenoxyl poly(ethyleneoxy)methacrylate, nonyl phenoxy poly(ethyleneoxy)crotanate, nonylphenoxy poly(ethyleneoxy)fumarate, nonyl phenoxy poly(ethyleneoxy)acrylate, nonylphenoxy poly(ethyleneoxy)methacrylate, mono dodecyl maleate, and allylsulfosuccinate derivatives.
Additionally, anionic reactive surfactants have been disclosed in Japanese Patent Publication No. 46-12472, Japanese Kokai Patent Publication No. 54-144317, Japanese Patent Publication No. 46-34894, Japanese Patent Publication No. 56-29657, Japanese Kokai Patent Publication No. 51-30285, U.S. Pat. No. 4,814,514, and U.S. Pat. No. 5,324,862 among others. A review of reactive surfactants may be found in “Reactive Surfactants in Emulsion Polymerization” Guyot, A. and Tauer, K., in Advances in Polymer Science, Vol III, Springer-Verlag, Berlin, pp 43-65.
None of these reactive surfactants incorporate more than one reactive moiety in their structure. In fact, the styrenated phenol-based materials disclosed in U.S. Pat. No. 4,814,514 prepared by the addition of allyl glycidyl ether (AGE) to surfactant base molecules such as hydroxyl-functional fatty alcohols or substituted phenols specifically limit the amount of AGE to 1.0 mole.